1. Field of the Invention
The invention relates to a method for the optimization of joint arthroplasty component design, and more particularly to a method for the optimization of shoulder arthroplasty component design through the use of computed tomography scan data.
2. Description of the Related Art
Various prostheses for the replacement of the shoulder joint are known. In one example shoulder prosthesis, the upper portion of the humerus is replaced by a humeral component including (i) a stem that extends into a bore formed within the humerus and (ii) a generally hemispherical head portion that is connected to the stem. The hemispherical head of the humeral component articulates with a complementary concave section of a glenoid component mounted within the glenoid cavity of the scapula. This type of shoulder prosthesis may be called a “primary” or “total” prosthesis. In another example shoulder prosthesis, often called a “reverse” or “inverted” prosthesis, the glenoid component includes a convex section that articulates with a complementary concave section of the head of the humeral component.
One alternative to total shoulder replacement is referred to as shoulder hemiarthroplasty. In one version of this procedure, the humeral head is replaced with a generally hemispherical head that may or may not include a connected stem. The glenoid cavity of the scapula is not replaced with a glenoid component, but may be refinished in a way that gives it a smooth surface and a shape which matches the generally hemispherical replacement head. Another version of this procedure can use a glenoid component with resurfacing of the humeral head.
Several deficiencies have been found in currently available shoulder arthroplasty systems including glenoid sizes (primary and reverse) and humeral sizes that are not based on the anatomic distribution. In addition, the advent of reverse arthroplasty for the treatment of proximal humerus fractures has also changed the requirements for an appropriate fracture stem. Specific design features are necessary to make the fracture stem appropriate for hemiarthroplasty and reverse arthroplasty use. Although resurfacing of the humerus has become popular, the designs are not based on an anatomic distribution. The instrumentation that is currently available is inadequate and may lead to significant malposition in version and inclination.
Prior magnetic resonance imaging and cadaveric studies of glenohumeral anatomy have been performed on shoulders without arthritis (Iannotti et al., “The normal glenohumeral relationships. An anatomical study of one hundred and forty shoulders”, J Bone Joint Surg Am. 1992; 74:491-500; Hertel et al., “Geometry of the proximal humerus and implications for prosthetic design”, J Shoulder Elbow Surg., July/August 2002, pp. 331-338; and Boileau et al., “The Three-Dimensional Geometry Of The Proximal Humerus—Implications For Surgical Technique And Prosthetic Design”, J Bone Joint Surg [Br], 1997; 79-B:857-865). However, in reality, shoulder arthroplasty is not performed on normal shoulders. Shoulder arthroplasty is performed in patients with arthritis in the setting of cartilage loss and usually associated bone loss. In order to make properly sized implants that will accommodate patients with arthritis, it is important to understand the anatomy of these patients.
Typically, the designing surgeon has used a system with three glenoid sizes. In one study, it was determined that the distribution of glenoid components used in total shoulder arthroplasty was as follows: 4% large, 40% medium, and 56% small. One can see that based on component use, the sizing of these implants is not optimal. If glenoid component sizes are not optimal, there may be issues related to perforation of the glenoid by fasteners used in attaching the glenoid component to the scapula. In addition, certain components may be too large for smaller patients resulting in component overhang and potentially leading to violation of important neurovascular structures. Thus, it could be hypothesized that the preference for small glenoid components may result from the desire to avoid glenoid perforation and/or avoid component overhang. However, larger glenoid components can lead to a better fitting prosthesis and greater stability.
There has been increasing interest in the use of augmented glenoid components in shoulder arthroplasty. Bone graft has been used in the past to manage bone deficiency; however there has been a high rate of graft resorption. It has also been clearly recognized that removal of the remaining hard cortical bone to create a neutral surface can compromise fixation by leaving the surgeon with only soft cancellous bone resulting in insufficient implant support for certain patients. In addition, excess reaming results in medialization and shortening the remaining rotator cuff lever arm with functional implications. Therefore, there has been increasing interest in the use of augmented glenoid components.
In FIG. 5A, one example augmented glenoid component 102 for use in a total shoulder system is shown. The glenoid component 102 has a single component plastic body 104. A concave articular surface 105 of the body 104 provides a smooth bearing surface for the head portion of the humeral component implanted into the humerus. The thickness of the plastic body 104 gradually increases from an anterior edge 106 to a posterior edge 108 thereof thereby creating a relatively smooth, arcuate-shaped medial base surface 110 from which a number of posts or pegs 112 extend. It can be seen that the augmented glenoid component 102 has an augment that has a defined slope along the entire posterior surface of the glenoid. An augment thickness can be defined as the thickness of the posterior edge 108 minus the thickness of the anterior edge 106.
In FIG. 5B, another example augmented glenoid component 114 for use in a total shoulder system is shown. The augmented glenoid component 114 includes a body 116 having a concave articular surface 118 on one end thereof. The concave surface 118 of the body 116 provides a smooth bearing surface for the head portion of the humeral component implanted into the humerus. The body 116 includes a step 120 on or from a body surface 122 opposite the concave surface 118. The step 121 forms a portion of the posterior edge 121 of the body 116. The augmented glenoid component 114 also includes an anchor peg 123 and a plurality of stabilizing posts pegs 124. It can be seen that the augmented glenoid component 114 has an augment that is a step on the posterior aspect of the glenoid. An augment thickness can be defined as the thickness of the posterior edge 121 minus the thickness of the anterior edge 117.
In FIGS. 6A and 6B, an example augmented glenoid component 130 for use in a reverse shoulder system is shown. The glenoid component 130 includes a baseplate 132 in which the thickness of the baseplate 132 gradually increases from a first edge 133 to an opposite second edge 134 thereof. The baseplate 132 has a surface 136 from which a peg 138 extends. The baseplate 132 is secured in a glenosphere 139 forming the glenoid component 130. The glenosphere 139 has an convex articular surface 137 that provides a smooth bearing surface for the concave articular portion of the humeral component implanted into the humerus. An augment thickness can be defined as the thickness of the second edge 134 minus the thickness of the first edge 133.
In FIGS. 6C and 6D, another example augmented glenoid component 130A for use in a reverse shoulder system is shown. The glenoid component 130A includes a baseplate 132A in which the thickness of the baseplate 132A gradually increases from a first edge 133A to an approximately central section and then the thickness is approximately constant to an opposite second edge 134A thereof. The baseplate 132A has a surface 136A from which a peg 138A extends. The baseplate 132A is secured in a glenosphere 139A forming the glenoid component 130A. The glenosphere 139A has an convex articular surface 137A that provides a smooth bearing surface for the concave articular portion of the humeral component implanted into the humerus. An augment thickness can be defined as the thickness of the first edge 134A minus the thickness of the second edge 133A.
However, significant deficiencies have been found in currently available augmented glenoid components that are not based on an anatomic distribution. The currently available commercial designs for augmented glenoids are not designed based on the specific dimensions of glenoid bone loss present in patients undergoing shoulder arthroplasty. In order to make properly sized augmented glenoid components that will accommodate patients with arthritis, it is important to understand the anatomy of these patients. One issue that continues to be raised is that no one has ever defined on average where this transition zone begins between native bone and worn bone. This would allow one to design an augment that is shaped according to the defects that actually exist and covers the appropriate amount of glenoid worn rather than being based on guesswork. Ideally, to design proper augmented glenoids one needs to define the bone loss based on the anatomy of patients actually undergoing shoulder arthroplasty. In order to make properly sized augmented glenoid components that will accommodate patients with arthritis, it is important to understand the anatomy of these patients.
Thus, there exists a need for a method for the optimization of joint arthroplasty component design, and in particular, there exists a need for a method for the optimization of shoulder arthroplasty component design.